Global Slow-mo effect - Animating Time scale
You can do it in the graph editor. Select all elements in the scene and place a locrot keyframe where you want the remapping to start. In the graph editor place the cursor on that frame and select all keyframes only to the right of the cursor. Change pivot point to 2d cursor and scale on x by 8 (s-x-8). Go to where you want the mapping to end and place another keyframe on all elements in the scene, place the cursor on that frame, then select all only to the right and scale on x by 0.125 (s-x-0.125). You then only need to finesse the handles on these keyframes to give you the lead in/lead out you're looking for. Much easier if the scene can jump straight to slo-mo because you can just set handle type to vector for those two frame numbers.
edit: video link https://youtu.be/lxdQogx6y40